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@QuestionOverflow You (and who upvoted your comment) should vote to close if you think it deserves to be closed.
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Alenanno♦Oct 23 '12 at 15:45

@Alenanno, I left a comment instead of voting to close because, to me, it isn't an exact duplicate, and this question might be useful to others. Unless the definition of an exact duplicate is changed to something else, I find it hard to vote using that option, especially when it is the only question of the day :D
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杨以轩Oct 24 '12 at 6:59

1

Just on a side note, saying 四亿二千 can totally be understood.
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deutschZuidOct 24 '12 at 22:11

Note to those who flagged this answer: I'm keeping it here for reference as it provides some relevant information.
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xiaohouzi79♦Oct 25 '12 at 19:19

I was taught the way you describe, @daodaoxx, but the Wikipedia reference claims that usage of 二 and 两 varies between dialects and even individual speakers within a dialect or region.
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Don Kirkby♦Oct 26 '12 at 4:23

Actually I agree with James' comment that "四亿二千 can totally be understood", and therefore 零 is optional. That is why I am providing an alternative answer.

Let's go back to the particular section on "Reading and transcribing numbers" in the Wikipedia article on Chinese numerals again. Below, I quote the following:

Interior zeroes before the unit position (as in 1002) must be spelt
explicitly. The reason for this is that trailing zeroes (as in 1200)
are often omitted as shorthand, so ambiguity occurs. One zero is
sufficient to resolve the ambiguity. Where the zero is before a digit
other than the units digit, the explicit zero is not ambiguous and is
therefore optional, but preferred.

There are a few things that need to be explained here:

Chinese numerals are grouped in blocks of four digits. So 400,002,000 can be regrouped as 4,0000,2000.

Unit position is the position occupied by the right most digit in each block. Therefore, the digits 4, the 4th 0, and the last 0 occupies the unit position of their respective blocks.

The middle block of 4,0000,2000 is completely comprised of 0s.

Whenever a zero precedes a non-zero digit occupying the unit
position within a block, this zero has to be transcribed explicitly. This is actually what the last sentence in the quoted paragraph is trying to say.

Each block can be transcribed independently of other blocks.

To clarify what point 4 means, let us take a look at a few examples:

4,2002,2000 is transcribed as 四亿 [二千零二万] 二千

4,2000,2000 is transcribed as 四亿 [二千万(零)] 二千

4,2022,2000 is transcribed as 四亿 [二千(零)二十二万] 二千

4,0202,0202 is transcribed as 四亿 [(零)二百零二万] (零)二百零二

4,2020,2020 is transcribed as 四亿 [二千(零)二十万] 二千(零)二十

() - represents optional zero
[] - represents the middle block

And to clarify what point 5 means, take a look at the examples below:

4,0000,0000 is transcribed as 四亿

400,0000 is transcribed as 四百万

40 is transcribed as 四十

Combining them is as simple as doing a concatenation:

四亿四百万四十 is interpreted as 4,0400,0040 only

Going back to the number 400,002,000. Since the middle block is all zeros, there is no need to transcribe the middle block. 四亿二千 is not ambiguous and 零 is added optionally to remind people that there is at least one zero between the digits 4 and 2.